There have been three different mold designs for the modern minifig torso since it was first introduced in 1978. Bricklink does not distinguish among these three types in its catalog, and many of them exist in different versions. If you are completing a set and want to have the appropriate version, you should look for sellers that indicate the type in their listings or ask before placing an order.
The earliest minifigs had hollow torsos. This version was produced from 1978 until 1982. Because LEGO often used up existing stock whenever making a design change, it is possible that hollow torsos were included in sets after 1982.
In 1983, a new version was introduced that had four wedge-shaped ridges running vertically inside the torso. These ridges provided more clutch power between the torso and hip pieces. This style of torso is often called the “short-prong” version.
Beginning in 1998 a third re-design of the torso was introduced. This version had slightly thinner front and back walls and longer X-shaped ribs. This is the version used at the present time.
At around the same time as the 1983 re-design, the color of the hands also changed slightly from a lighter, more translucent yellow color to the more solid and slightly darker yellow used today. This color change is the same one that took place with other minifig-related parts such as airtanks. Hollow-torso minifigs prior to 1983 should have the light yellow hands, with some exceptions – the first red classic space astronauts had hollow torsos, but red hands for example. Also, the very first minifigs from 1978 had darker, more orange-yellow hands.
The torso type is a quick way to tell re-issued torsos from originals, or to date a minifig to a specific range of production years. For example, the classic patch-pocket button shirt torso shown in the but015 figure above has been produced in all three torso versions.